Primary headaches are debilitating ailments that afflict millions of individuals worldwide. The specific pathophysiology of primary headaches is not known. Known causes of headache pain include trauma, vascular defects, autoimmune deficiencies, degenerative conditions, infections, drug and medication-induced causes, inflammation, neoplastic conditions, metabolicendocrine conditions, iatrogenic conditions, musculoskeletal conditions, and myofacial causes. In many situations, however, even though the underlying cause of the headache may be identified and treated, the headache pain itself may persist.
Recent clinical studies in treatment of headaches have targeted the manipulation of sphenopalatine (pterygopalatine) ganglion (SPG), a large, extra-cranial parasympathetic ganglion. A ganglion is a mass of neural tissue found in some peripheral and autonomic nerves. Ganglia are located on the roots of the spinal nerves and on the roots of the trigeminal nerve. Ganglia are also located on the facial, glossopharyngeal, vagus and vestibulochoclear nerves. The SPG is a complex neural ganglion with multiple connections, including autonomic, sensory, and motor connections. The SPG includes parasympathetic neurons that innervate, in part, the middle cerebral and anterior cerebral blood vessels, the facial blood vessels, and the lacrimal glands.
The maxillary branch of the trigeminal nerve and the nerve of the pterygoid canal (also known as the vidian nerve which is formed by the greater and deep petrosal nerves) send neural projections to the SPG. The fine branches from the maxillary nerve (pterygopalatine nerves) form the sensory component of the SPG. These nerve fibers pass through the SPG and do not synapse. The greater petrosal nerve carries the preganglionic parasympathetic axons from the superior salivary nucleus, located in the pons, to the SPG. These fibers synapse onto the postganglionic neurons within the SPG. The deep petrosal nerve connects the superior cervical sympathetic ganglion to the SPG and carries postganglionic sympathetic axons that again pass through the SPG without any synapsing in the SPG.
The SPG is located within the pterygopalatine fossa. The pterygopalatine fossa is bounded anteriorly by the maxilla, posteriorly by the medial plate of the pterygoid process and greater wing of the sphenoid process, medially by the palatine bone, and superiorly by the body of the sphenoid process. The lateral border of the pterygopalatine fossa is the pterygomaxillary fissure, which opens to the infratemporal fossa. Various clinical approaches have been used to modulate the function of the SPG in order to treat headaches, such as cluster headaches or chronic migraines. These approaches vary from lesser or minimally invasive procedures (e.g., transnasal anesthetic blocks) to procedures or greater invasiveness (e.g., surgical ganglionectomy). Other procedures of varying invasiveness include those such as surgical anesthetic injections, ablations, gamma knife procedures, and cryogenic surgery. Although most of these procedures can exhibit some short term efficacy in the order of days to months, the results are usually temporary and the headache pain eventually reoccurs.